With the use of any form of electrical appliance, there is a need to electrically insulate conductors. With the push to continuously reduce the size and to streamline all electrical and electronic systems, there is a corresponding need to find better and more compact insulators and insulation systems.
Because they have the practical benefit of being tough and flexible electrical insulation materials that can be easily adhered to surfaces, various epoxy resin materials have been used extensively in electrical insulation systems. Traditional electrical insulation materials, such as mica flake and glass fiber, can be surface coated and bonded with these epoxy resins, to produce composite materials with increased mechanical strength, chemical resistance and electrical insulating properties. Epoxy resins frequently replace traditional varnishes, although some high voltage equipment continues to utilize such materials.
Because of their nature, materials which are good electrical insulators are inherently good thermal insulators, which, is undesirable. Thermal insulating behavior, particularly for air-cooled electrical equipment and components, reduces the efficiency and durability of the components as well as the equipment as a whole. The production of electrical insulation systems having maximum electrical insulation and minimal thermal insulation characteristics is desirable.
Electrical insulation often appears in the form of insulating tapes, which themselves have various layers. Common to these types of tapes is a paper layer that is bonded at an interface to a fiber layer, both layers tending to be impregnated with a resin. A favored type of insulation material is a mica-tape. Improvements to mica tapes include catalyzed mica tapes as taught in U.S. Pat. No. 6,103,882. The mica-tape may be wound around conductors to provide extremely good electrical insulation. An example of this is shown in FIG. 1. Illustrated here is a coil 13, comprising a plurality of turns of conductors 14, which in the example illustrated here are assembled into a bakelized coil. The turn insulation 15 is prepared from a fibrous material, for example glass or glass and Dacron which is heat treated. Ground insulation for the coil is provided by wrapping one or more layers of composite mica tape 16 about the bakelized coil 14. Such composite tape may be a paper or felt of small mica flakes combined with a pliable backing sheet 18 of, for example, glass fiber cloth or polyethylene glycol terephthalate mat, the layer of mica 20 being bonded thereto by a liquid resinous binder. Generally, a plurality of layers of the composite tape 16 are wrapped about the coil depending upon voltage requirements. A wrapping of an outer tape 21 of a tough fibrous material, for example, glass fiber, may be applied to the coil.
Generally, multiple layers of the mica tape 16 are wrapped about the coil -with sixteen or more layers generally being used for high voltage coils. Resins are then impregnated into the tape layers. Resins can even be used as insulation independently from the insulating tape. Unfortunately this amount of insulation only further adds to the complications of dissipating heat.
During service in high voltage electrical equipment, microvoids can be created during resin cure and interfacial delamination because mica has poor wetting and adhesion with the impregnating resin, such as epoxy. The micro pores within the mica are particularly poor for wetting and adhesion of resin to since they are deep within the mica paper. The primary insulator is mica in the form of flakes or platelets, which are then used in splittings or paper. During winding and subsequent processing of the insulation, mica tapes have adequate mechanical strength, but poor wetting.
The poor wetting characteristics make it difficult to get the impregnating resin, and fillers within the resin, to penetrate and adhere to these micropore areas of the mica. This poor wetting of impregnating resin and fillers may cause air gaps in the structure which reduce thermal conductivity, consequently lowering thermal conduction properties in the mica insulation. Additionally, this poor wetting results in microvoid formation, which in turn causes partial discharges under high voltage within the insulation structure. This results in inferior voltage endurance, thereby reducing the service lifetime of the electrical equipment.
What is needed are better techniques and approaches to getting resins to adhere to the inner surfaces of the . mica micropores. Other difficulties with the prior art also exist, some of which will be apparent upon further reading.